Transmission mechanism

ABSTRACT

A transmission mechanism includes a plurality of drive gears disposed on a counter shaft and a plurality of driven gears disposed on a drive shaft. Each of the plurality of drive gears is engaged with each of the plurality of driven gears to form a plurality of shift-gear pairs. An eight-speed drive gear, which forms an eight-speed shift-gear pair having the smallest reduction gear ratio, and a seven-speed drive gear, which forms a seven-speed shift-gear pair having the second smallest reduction gear ratio, are disposed adjacent one another at a center portion of an arrangement of the drive gears. An eight-speed driven gear, which forms the eight-speed shift-gear pair, and a seven-speed driven gear, which forms the seven-speed shift-gear pair, are disposed adjacent one another at a center portion of an arrangement of the driven gears.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-022652, filed on Feb. 9,2016, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a transmission mechanism.

Description of the Related Art

As a transmission mechanism for vehicle, a constant-mesh typetransmission mechanism has been known. Patent Document 1 discloses aconstant-mesh type transmission mechanism including two shafts: acounter shaft and a drive shaft. In such transmission mechanism, aplurality of driving gears are disposed on the counter shaft whererotative power is transmitted from a crankshaft, and a plurality ofdriven gears are disposed on the drive shaft that outputs the rotativepower. Each of the plurality of driving gears is constantly engaged witheach of the plurality of driven gears. Then, changing a transmissionroute of the rotative power from the counter shaft to the drive shaftshifts gears.

A six-speed is widely used for such constant-mesh type transmissionmechanism. However, a further multistage is required in order to ensureboth improvement of fuel efficiency during high-speed running andsecurement of acceleration performance during acceleration. As aconfiguration to ensure the multistage of the constant-mesh typetransmission mechanism, Patent Document 2 discloses a configuration thatadds a two-speed auxiliary transmission mechanism to a five-speed maintransmission mechanism.

Patent Document 1: Japanese Laid-open Patent Publication No. 2005-127392

Patent Document 2: Japanese Laid-open Patent Publication No. 61-189347

However, in the configuration that adds the auxiliary transmissionmechanism, it is necessary to add a rotation shaft for adjusting arotation direction of the output rotative power and a mechanism foroperating the auxiliary transmission mechanism, thus the configurationbecomes complicated. On the other hand, in order to ensure the furthermultistage in the transmission mechanism including the two shafts: thecounter shaft and the drive shaft, it is necessary to increase thenumber of the driving gears disposed on the counter shaft and the numberof the driven gears disposed on the drive shaft. Therefore, it isnecessary to extend the counter shaft and the drive shaft, thusdeflections at center portions in longitudinal directions of the countershaft and the drive shaft become large.

SUMMARY OF THE INVENTION

In consideration of the above-described actual condition, the object ofthe present invention is to ensure the further multistage in thetransmission mechanism including the two shafts: the counter shaft andthe drive shaft, while suppressing deformation of the counter shaft andthe drive shaft.

To achieve the above-described object, the present invention includes aninput shaft to which a rotative power is transmitted, a plurality ofdrive gears arranged in an axial direction on the input shaft, an outputshaft that outputs a rotative power outside, and a plurality of drivengears arranged in an axial direction on the output shaft, and each ofthe plurality of drive gears is engaged with each of the plurality ofdriven gears so as to ensure transmission of the rotative power to forma plurality of shift-gear pairs having mutually different reduction gearratios, a first drive gear included in a first shift-gear pair and asecond drive gear included in a second shift-gear pair are adjacent oneanother at a center portion of an arrangement of the plurality of drivegears, the first shift-gear pair has a smallest reduction gear ratioamong the plurality of shift-gear pairs, and the second shift-gear pairhas a small reduction gear ratio next to the first shift-gear pair, anda first driven gear included in the first shift-gear pair and a seconddriven gear included in the second shift-gear pair are adjacent oneanother at a center portion of an arrangement of the plurality of drivengears.

The present invention may have a configuration where the plurality ofdrive gears include slide gears reciprocatable in the axial direction ofthe input shaft, and free gears disposed adjacent to the slide gears inthe axial direction of the input shaft and rotatable relative to theinput shaft, the plurality of driven gears include slide gearsreciprocatable in the axial direction of the output shaft, and freegears disposed adjacent to the slide gears in the axial direction of theoutput shaft and rotatable relative to the output shaft, convex portionsprojecting from end surfaces in the axial direction of teeth aredisposed on ones of the slide gears and the free gears included in theplurality of drive gears and adjacent one another, hollow portionsdepressing from end surfaces in the axial direction of teeth aredisposed on others, and the convex portions are fitted into the hollowportions to couple the slide gears and the free gears included in theplurality of drive gears and adjacent one another so as to ensure thetransmission of the rotative power, and convex portions projecting fromend surfaces in the axial direction of teeth are disposed on ones of theslide gears and the free gears included in the plurality of driven gearsand adjacent one another, hollow portions depressing from end surfacesin the axial direction of teeth are disposed on others, and the convexportions are fitted into the hollow portions to couple the slide gearsand the free gears included in the plurality of driven gears andadjacent one another so as to ensure the transmission of the rotativepower.

The present invention may have a configuration where each of theplurality of slide gears included in the plurality of drive gears andthe plurality of driven gears is adjacent to two free gears havingmutually different outside diameters among the plurality of free gears,a hollow portion is disposed on a free gear having a large diameter ofthe two free gears, the hollow portion ensures insert of an end portionin an axial direction of the slide gear adjacent to the free gear havingthe large diameter, and when the free gear having the large diameter iscoupled to the slide gear adjacent to the free gear having the largediameter so as to ensure the transmission of the rotative power, a partof the free gear having the large diameter is superimposed on a part ofthe slide gear adjacent to the free gear having the large diameter,viewed from a direction perpendicular to a rotational center line.

The present invention may have a configuration where differences ofcounts of shift stages of the free gears having the large diameters andcounts of shift stages of the slide gears adjacent to the free gearshaving the large diameter are all identical.

The present invention may have a configuration where the plurality ofdrive gears further include a plurality of fixed gears that arenonmovable in the axial direction with respect to the input shaft andintegrally rotate with the input shaft, and the free gears, the slidegears, and the fixed gears included in the plurality of drive gears aresymmetrically disposed about a center of the arrangement of theplurality of drive gears.

The present invention may have a configuration where the plurality ofdriven gears further include a plurality of fixed gears that arenonmovable in the axial direction with respect to the output shaft andintegrally rotate with the output shaft, and the free gears, the slidegears, and the fixed gears included in the plurality of driven gears aresymmetrically disposed about a center of the arrangement of theplurality of driven gears.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a right side view schematically illustrating an exemplaryconfiguration of a motorcycle;

FIG. 2 is a perspective view schematically illustrating an exemplaryconfiguration of a transmission mechanism;

FIG. 3 is a perspective view schematically illustrating the exemplaryconfiguration of the transmission mechanism;

FIG. 4 is a cross-sectional view schematically illustrating theexemplary configuration of the transmission mechanism;

FIG. 5 is a table that summarizes an arrangement of shift-gear pairs,and types of drive gears and driven gears that form the shift-gearpairs;

FIG. 6A is a cross-sectional view schematically illustrating anexemplary configuration of an exemplary set of slide/free gears;

FIG. 6B is a cross-sectional view schematically illustrating anexemplary configuration of an exemplary set of slide/free gears; and

FIG. 7 is a cross-sectional view schematically illustrating an exemplarypairs of shift forks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes an embodiment of the present invention in detailwith reference to the drawings. The embodiment of the present inventionshows an example where a transmission mechanism is applied to an engineunit of a motorcycle. For convenience of explanation, each direction ofthe motorcycle is based on a direction viewed from an occupant thatrides the motorcycle. In each drawing, as necessary, at the motorcycle,a front side is indicated by an arrow Fr, a rear side is indicated by anarrow Rr, a right side is indicated by an arrow R, a left side isindicated by an arrow L, an upper side is indicated by an arrow Up, anda downside is indicated by an arrow Dn.

Overall Configuration of Motorcycle

First, a description will be given of an exemplary overall configurationof a motorcycle 1 where a transmission mechanism 5 according to theembodiment is applied, with reference to FIG. 1. FIG. 1 is a right sideview schematically illustrating the exemplary configuration of themotorcycle 1.

A vehicle body frame 11 of the motorcycle 1 includes a steering headpipe 111 and aright and left pair of main frames 112. The steering headpipe 111 has a tubular configuration that inclines rearward. The rightand left pair of main frames 112 are integrally bonded on the steeringhead pipe 111 at front end portions and enlarge a distance in avehicle-width direction from the steering head pipe 111 to extendrearward and obliquely downward. Aright and left pair of seat rails 113are mounted on rear portions of the main frames 112. The right and leftpair of seat rails 113, which are members that support a seat 141,extend from the rear portions of the main frames 112 rearward andobliquely upward at a predetermined distance in the vehicle-widthdirection. Respective portions of the vehicle body frame 11, which aremade of, for example, a steel material or an aluminum alloy material,are integrally bonded by welding or similar method.

A steering shaft (not illustrated in hiding in FIG. 1), a right and leftpair of front forks 121, and a front wheel 122 are disposed on a frontside of the vehicle body frame 11. The steering shaft is inserted intothe steering head pipe 111 to be rotatably supported by the steeringhead pipe 111. The right and left pair of front forks 121 are coupled tothe steering shaft via, for example, a bracket to rotate integrally withthe steering shaft. The front wheel 122 is rotatably supported by lowerend portions of the right and left pair of front forks 121. A brake disc123, which rotates integrally with the front wheel 122, is mounted onthe front wheel 122. A brake caliper 124 of the front wheel 122, and afront fender 125, which covers an upper side of the front wheel 122, aremounted on the right and left pair of front forks 121. A right and lefthandlebars 126 (handle grips) are mounted on each upper end portion ofthe right and left pair of front forks 121. A clutch lever for operatinga clutch 26 is disposed on the left side handlebar 126. A brake leverthat operates a brake of a rear wheel 132 is disposed on the right sidehandlebar 126. A shift lever for the occupant operating the transmissionmechanism 5 is disposed on a left side lower portion in thevehicle-width direction of the vehicle body frame 11.

A swing arm 131 is coupled to a rear portion of the vehicle body frame11. The swing arm 131 is swingable in a vertical direction (a pitchingdirection). The rear wheel 132, which is an exemplary driving wheel, isrotatably supported by a rear end portion of the swing arm 131. A drivensprocket 133, which rotates integrally with the rear wheel 132, ismounted on a left side of the rear wheel 132. A drive chain 134 is woundacross the driven sprocket 133 and a drive sprocket 30 (described below)of an engine unit 2. Then, the rotative power output by the engine unit2 is transmitted to the rear wheel 132 via the drive sprocket 30 and thedrive chain 134. A shock absorber (not illustrated) is disposed betweenthe vehicle body frame 11 and the swing arm 131. This shock absorberabsorbs and mitigates oscillation and impact transmitted from the rearwheel 132 to the vehicle body frame 11. Additionally, a rear fender 135is disposed above the rear wheel 132.

The seat 141 where the occupants (a rider and a pillion passenger) areseated is disposed on upper sides of the seat rails 113. A fuel tank 142is disposed on a front side of the seat 141 and upper sides of the mainframes 112. Additionally, the motorcycle 1 includes a front cowl 143,side cowls 144, and a seat cowl 145, as exterior members. The front cowl143 covers a front portion the motorcycle 1. The side cowls 144 coverside portions of the motorcycle 1. The seat cowl 145 covers a peripheryof the seat 141. Testaceous members made of, for example, syntheticresin material are applied to these exterior members. These exteriormembers constitute an exterior design of the motorcycle 1.

The engine unit 2 includes an engine portion (an internal-combustionportion), which is a driving force source of the motorcycle 1, thetransmission mechanism 5, which shifts gears of the rotative power ofthe engine portion to output, and the clutch 26, which engages anddisengages transmission of the rotative power between the engine portionand the transmission mechanism 5. These members are integrally attached.Then, the engine unit 2 is suspended on the vehicle body frame 11. Theengine unit 2 has also a function of a strength member of the motorcycle1. The engine unit 2 includes a crankcase 21, a cylinder block 22, acylinder head 23, and a cylinder head cover 24.

A crank chamber 211 is disposed nearer a front side of an inside of thecrankcase 21. A transmission chamber 212 is disposed nearer a rear sideof the inside of the crankcase 21. A clutch cover 25 is mounted on aright side surface of the crankcase 21. A magneto cover is mounted on aleft side surface of the crankcase 21.

A crankshaft 29 is rotatably housed in the crank chamber 211 such thatan axis line (a rotational center line) of the crankshaft 29 is in adirection parallel to a lateral direction (the vehicle-width direction).A primary drive gear 27 is disposed on one end portion (a right side endportion in the embodiment) in an axial direction of the crankshaft 29,as the primary drive gear 27 rotates integrally with the crankshaft 29.The primary drive gear 27 is a gear for transmitting the rotative powerto the transmission mechanism 5, which is described later. A magneto iscoupled to the other end portion (a left side end portion in theembodiment) in the axial direction of the crankshaft 29. The magneto isa generator that generates electricity by the rotative power transmittedfrom the crankshaft 29. The magneto is covered with the magneto covermounted on the left side surface of the crankcase 21.

The transmission mechanism 5 and the clutch 26 are disposed inside thetransmission chamber 212. The transmission mechanism 5 shifts gears ofthe rotative power input from the crankshaft 29 to output to the rearwheel 132, which is the exemplary driving wheel. A configuration of thetransmission mechanism 5 will be described later in detail. The clutch26 engages and disengages the transmission of the rotative power betweenthe engine portion and the transmission mechanism 5. Various knownclutches, such as a multiplate wet clutch, are applicable to the clutch26. The clutch 26 is disposed on one end portion (a right side endportion in the embodiment) in an axial direction of the counter shaft51, as being coaxial with the counter shaft 51. Then, the clutch 26 iscovered with the clutch cover 25 mounted on the right side surface ofthe crankcase 21.

The cylinder block 22 is disposed on an upper side nearer a front side(that is, a part where the crank chamber 211 is disposed) of thecrankcase 21. Predetermined number of (for example, four) combustionchambers (cylinders) are disposed inside the cylinder block 22, forexample, as being aligned in the lateral direction (the vehicle-widthdirection). A piston is reciprocatably housed in each combustionchamber. Each piston is coupled to the crankshaft 29 by a connectingrod. The cylinder head 23 is disposed on an upper side of the cylinderblock 22. The cylinder head 23 includes intake ports, which are intakepaths, and exhaust ports, which are exhaust paths, for each of thepredetermined number of combustion chambers. Further, the cylinder head23 includes intake valves, which open and close the intake ports,exhaust valves, which open and close the exhaust ports, and a valvedrive mechanism, which drives the intake valves and the exhaust valves.The cylinder head cover 24 is disposed on an upper side of the cylinderhead 23. The cylinder head cover 24 covers the intake valves, theexhaust valve, the valve drive mechanism, and similar member included inthe cylinder head 23.

An air cleaner is disposed above the engine unit 2. The air cleanerpurifies combustion air that the engine unit 2 uses. The air cleaner iscoupled to the respective intake ports via intake paths that ensure passof the combustion air. The respective intake paths include throttlebodies that adjust flow rates of the combustion air.

An exhaust pipe, which is a passage of exhaust gas, is coupled to theexhaust ports included in the cylinder head 23. Then, a muffler iscoupled to a rear end of the exhaust pipe. The exhaust gas from therespective combustion chambers is discharged outside through the exhaustports, the exhaust pipe, and the muffler.

Transmission Mechanism

The following describes an exemplary configuration of the transmissionmechanism 5. FIG. 2 and FIG. 3 are perspective views schematicallyillustrating the exemplary configuration of the transmission mechanism5. The transmission mechanism 5 is a constant-mesh type. The embodimentshows an example where a total number of shift stages of thetransmission mechanism 5 is eight. As illustrated in FIG. 2 and FIG. 3,the transmission mechanism 5 includes two transmission shafts: a countershaft 51 and a drive shaft 52, and transmission gears: predeterminednumber of (but, a plurality of) drive gears 53-1 to 53-8 andpredetermined number of (a plurality of) driven gears 54-1 to 54-8.Furthermore, the transmission mechanism 5 includes a shift cam 56,predetermined number of shift forks 57, and two shift fork guides 58,which movably support the shift forks 57.

The counter shaft 51 is an input shaft where the rotative power istransmitted from an outside of the transmission mechanism 5 (thecrankshaft 29). The drive shaft 52 is an output shaft that transmits(outputs) the rotative power to the outside (the rear wheel 132, whichis the exemplary driving wheel) of the transmission mechanism 5. Thecounter shaft 51 and the drive shaft 52 are rotatably housed in thetransmission chamber 212 in a direction where axis lines (rotationalcenter lines) of the counter shaft 51 and the drive shaft 52 areparallel one another and parallel to the lateral direction (thevehicle-width direction). Then, the counter shaft 51 and the drive shaft52 are rotatably supported on the crankcase 21 by bearings 511R and511L, and 521R and 521L (Not illustrated in FIG. 2 and FIG. 3. See FIG.4 and FIG. 7.).

A primary driven gear 28 and the clutch 26 are disposed at the proximityof the right side end portion of the counter shaft 51. The primarydriven gear 28 is coaxial with the counter shaft 51 and rotatablerelative to the counter shaft 51. The primary driven gear 28 is engagedwith the primary drive gear 27 disposed on the crankshaft 29 to rotatesuch that the rotative power from the engine portion (the crankshaft 29)is transmitted. The clutch 26 engages and disengages the transmission ofthe rotative power between the primary driven gear 28 and the countershaft 51. Thus, the rotative power from the crankshaft 29 is transmitted(input) to the counter shaft 51, which is the input shaft of thetransmission mechanism 5, via the primary drive gear 27, the primarydriven gear 28, and the clutch 26. Thus, the counter shaft 51 functionsas the input shaft where the rotative power is transmitted (input) fromthe outside (the engine portion) of the transmission mechanism 5.

One end part of the drive shaft 52 projects from the left side surfaceof the crankcase 21 to an outside of the crankcase 21. The drivesprocket 30 is disposed on this projecting part, as the drive sprocket30 integrally rotates with the projecting part. The drive chain 134 iswound across the drive sprocket 30 and the driven sprocket 133 of therear wheel 132. Then, the rotative power from the drive shaft 52 istransmitted to the rear wheel 132 via the drive chain 134. Thus, thedrive shaft 52 functions as the output shaft that transmits (outputs)the rotative power to the outside (the rear wheel 132, which is thedriving wheel) of the transmission mechanism 5.

The eight drive gears 53-1 to 53-8, whose number is equal to the totalnumber of shift stages of the transmission mechanism 5, are arranged inthe axial direction of the counter shaft 51, on the counter shaft 51.

The eight drive gears 53-1 to 53-8 include fixed gears secured as beingnonmovable in the axial direction with respect to the counter shaft 51.The fixed gears rotate integrally with the counter shaft 51. Forexample, a configuration integrally disposed on the counter shaft 51 ora configuration secured to the counter shaft 51 by, for example, a keyis applied to the fixed gears included in the drive gears 53-1 to 53-8.The drive gears 53-1 to 53-8 include slide gears reciprocatable in theaxial direction with respect to the counter shaft 51. The slide gearsrotate integrally with the counter shaft 51. For example, aconfiguration splined to the counter shaft 51 is applied to the slidegears included in the drive gears 53-1 to 53-8. The drive gears 53-1 to53-8 include free gears nonmovable in the axial direction with respectto the counter shaft 51. However, the free gears are rotatable relativeto the counter shaft 51. The free gears included in the drive gears 53-1to 53-8 are rotatably supported on the counter shaft 51 by a bearingsuch as a needle bearing.

The eight driven gears 54-1 to 54-8 include fixed gears secured as beingnonmovable in an axial direction with respect to the drive shaft 52. Thefixed gears rotate integrally with the drive shaft 52. The driven gears54-1 to 54-8 include slide gears reciprocatable in the axial directionwith respect to the drive shaft 52. The slide gears rotate integrallywith the drive shaft 52. The driven gears 54-1 to 54-8 include freegears nonmovable in the axial direction with respect to the drive shaft52. However, the free gears are rotatable relative to the drive shaft52. Configurations of the fixed gears, the slide gears, and the freegears included in the driven gears 54-1 to 54-8 may be identical to theconfigurations of the fixed gears, the slide gears, and the free gearsincluded in the drive gears 53-1 to 53-8 respectively.

The eight driven gears 54-1 to 54-8, whose number is equal to the totalnumber of shift stages of the transmission mechanism 5, are arranged inthe axial direction of the drive shaft 52, on the drive shaft 52. Theeight driven gears 54-1 to 54-8, similarly to the eight drive gears 53-1to 53-8, include two fixed gears, two slide gears, and four free gears.The fixed gears of the eight driven gears 54-1 to 54-8 are nonmovable inthe axial direction with respect to the drive shaft 52 and rotateintegrally with the drive shaft 52. The slide gears of the driven gears54-1 to 54-8 are movable in the axial direction with respect to thedrive shaft 52 and rotate integrally with the drive shaft 52. The freegears of the driven gears 54-1 to 54-8 are nonmovable in the axialdirection with respect to the drive shaft 52. However, the free gearsare rotatable relative to the drive shaft 52.

Each of the eight drive gears 53-1 to 53-8 is constantly engaged witheach of the eight driven gears 54-1 to 54-8 so as to ensure thetransmission of the rotative power. For convenience of explanation,pairs of the drive gears 53-1 to 53-8 and the driven gears 54-1 to 54-8being engaged one another are referred to as “shift-gear pairs.” Thetransmission mechanism 5 includes eight shift-gear pairs 55-1 to 55-8,whose number is equal to the total number of shift stages.

A cylindrical cam having an approximately cylindrical shape is appliedto the shift cam 56. The shift cam 56 is disposed in a direction whereits axis line (rotational center line) is parallel to the axis lines ofthe counter shaft 51 and the drive shaft 52. Four cam grooves 561, whosenumber is equal to the total number of (four in the embodiment) slidegears included in the drive gears 53-1 to 53-8 and the driven gears 54-1to 54-8, are disposed on aside surface of the shift cam 56.

The two shift fork guides 58, which are both approximately rod-shapedmembers, are disposed in a direction where these longitudinal directionsare parallel to the axial directions (the lateral directions) of thecounter shaft 51 and the drive shaft 52. One of the two shift forkguides 58 supports two shift forks 57, whose number is equal to thetotal number of (two in the embodiment) slide gears included in thedrive gears 53-1 to 53-8, reciprocatably in the longitudinal direction(the lateral direction). Similarly, the other shift fork guide 58supports two shift forks 57, whose number is equal to the total numberof (two in the embodiment) slide gears included in the driven gears 54-1to 54-8, reciprocatably in the longitudinal direction (the lateraldirection). Each of the four shift forks 57 is engaged with each of thefour cam grooves 561 disposed on the shift cam 56. Then, each of the twoshift forks 57 supported by the one shift fork guide 58 is engaged witheach of the two slide gears included in the drive gears 53-1 to 53-8.Each of the two shift forks 57 supported by the other shift fork guide58 is engaged with each of the two slide gears included in the drivengears 54-1 to 54-8. Then, each of the four shift forks 57 moves each ofthe four slide gears in the axial directions of the counter shaft 51 andthe drive shaft 52 corresponding to rotation of the shift cam 56.

Additionally, the transmission mechanism 5 includes a shift cam drivingmechanism, which rotates the shift cam 56 corresponding to operation ofthe shift lever by the occupant, and a shift holding mechanism, whichholds a rotation direction position of the shift cam 56. The shift camdriving mechanism transmits movement of the shift lever to the shift cam56 to rotate the shift cam 56. The shift cam driving mechanism may havea configuration that includes a sensor, which detects the movement ofthe shift lever, and an actuator such as a motor, which rotates theshift cam 56 to cause the actuator to rotate the shift cam 56corresponding to the movement of the shift lever detected by the sensor.In short, it is only necessary that the shift cam driving mechanism isconfigured to rotate the shift cam 56 corresponding to the operation ofthe shift lever by the occupant.

Configuration of Drive Gear and Driven Gear

The following describes an exemplary configuration of the drive gears53-1 to 53-8 and the driven gears 54-1 to 54-8 with reference to FIG. 4.FIG. 4 is a cross-sectional view schematically illustrating theexemplary configuration of the transmission mechanism 5, and a drawingschematically illustrating a cross-sectional surface where thetransmission mechanism 5 is cut off at a plane including the axis lineof the counter shaft 51 and the axis line of the drive shaft 52.

As illustrated in FIG. 4, the counter shaft 51 and the drive shaft 52are rotatably supported on the crankcase 21 by the two bearings 511R and511L, and the two bearings 521R and 521L respectively. The two bearings511R and 511L and the two bearings 521R and 521L are disposed separatelyat a predetermined distance in the axial direction. Then, the eightdrive gears 53-1 to 53-8, whose number is equal to the total number ofshift stages of the transmission mechanism 5, are arranged in the axialdirection (the lateral direction) between these two bearings 511R and511L on the counter shaft 51. Similarly, the eight driven gears 54-1 to54-8, whose number is equal to the total number of shift stages of thetransmission mechanism 5, are arrange in the axial direction (thelateral direction) between these two bearings 521R and 521L on the driveshaft 52. With such configuration, among the eight drive gears 53-1 to53-8 and the eight driven gears 54-1 to 54-8, two drive gears 53-7 and53-8 and two driven gears 54-7 and 54-8, which are positioned at centerportions of arrangements, are the farthest from the bearings 511R, 511L,521R, and 521L, and two drive gears 53-1 and 53-2 and two driven gears54-1 and 54-2, which are positioned at both end portions of thearrangements, are the closest to the bearings 511R, 511L, 521R, and521L.

It is only necessary that the transmission mechanism 5 has aconfiguration where the eight drive gears 53-1 to 53-8 are disposedbetween the two bearings 511R and 511L, and may have a configurationwhere other positions of the counter shaft 51 are support by bearingsdifferent from the two bearings 511R and 511L. Similarly, it is onlynecessary that the transmission mechanism 5 has a configuration wherethe eight driven gears 54-1 to 54-8 are disposed between the twobearings 521R and 521L, and may have a configuration where otherpositions of the drive shaft 52 are supported by bearings different fromthe two bearings 521R and 521L. However, the bearing is not disposedbetween the respective eight drive gears 53-1 to 53-8. Similarly, thebearing is not disposed between the respective eight driven gears 54-1to 54-8. That is, among the eight drive gears 53-1 to 53-8, each of thetwo drive gears 53-1 and 53-2, which are positioned at both ends of thearrangement, is adjacent to each of the two bearings 511R and 511L. Theother six drive gears 53-3 to 53-8 are respectively adjacent to theothers of the drive gears 53-1 to 53-8, and not adjacent to the bearings511R and 511L. The same applies to the driven gears 54-1 to 54-8.

Each of the eight drive gears 53-1 to 53-8 is constantly engaged witheach of the eight driven gears 54-1 to 54-8 so as to ensure thetransmission of the rotative power. This forms the eight shift-gearpairs 55-1 to 55-8 constituted of the drive gears 53-1 to 53-8 and thedriven gears 54-1 to 54-8, which are engaged one another. The eightshift-gear pairs 55-1 to 55-8 are each corresponding to one-speed toeight-speed shift stages (shift positions). For reduction gear ratios ofthe eight shift-gear pairs 55-1 to 55-8, the one of the eight-speedshift-gear pair 55-8 is smallest, the ones of the seven-speed shift-gearpair 55-7, the six-speed shift-gear pair 55-6, the five-speed shift-gearpair 55-5, the four-speed shift-gear pair 55-4, the three-speedshift-gear pair 55-3, and the two-speed shift-gear pair 55-2 increase inthis order, and the one of the one-speed shift-gear pair 55-1 islargest.

Then, the eight drive gears 53-1 to 53-8 and the eight driven gears 54-1to 54-8 are disposed as being aligned in predetermined orders in theaxial directions, on the counter shaft 51 and the drive shaft 52respectively. In view of this, the eight shift-gear pairs 55-1 to 55-8also are aligned in the predetermined order in the axial directions ofthe counter shaft 51 and the drive shaft 52. Here, the arrangement ofthe eight shift-gear pairs 55-1 to 55-8 will be described. The order ofthe arrangement of the eight drive gears 53-1 to 53-8 and the order ofthe arrangement of the eight driven gears 54-1 to 54-8 are identical tothat of the eight shift-gear pairs 55-1 to 55-8. For convenience ofexplanation, a center position (a position between the fourth and fifthshift-gear pairs from an end) of the arrangement of the eight shift-gearpairs 55-1 to 55-8 may be referred to as an “arrangement center.” InFIG. 4, FIG. 6A, FIG. 6B, and FIG. 7, the arrangement center isindicated by a dot-and-dash line C. The same applies to the eight drivegears 53-1 to 53-8 and the eight driven gears 54-1 to 54-8.

As illustrated in FIG. 4, the eight-speed shift-gear pair 55-8 havingthe smallest reduction gear ratio and the seven-speed shift-gear pair55-7 having the second smallest reduction gear ratio are disposed asbeing adjacent one another at the center portion of the arrangement ofthe eight shift-gear pairs 55-1 to 55-8. That is, the eight-speedshift-gear pair 55-8 having the smallest reduction gear ratio and theseven-speed shift-gear pair 55-7 having the second smallest reductiongear ratio are disposed as being adjacent one another with sandwichingthe arrangement center C. FIG. 4 illustrates an exemplary configurationwhere the eight-speed shift-gear pair 55-8 having the smallest reductiongear ratio is positioned nearer the right side, and the seven-speedshift-gear pair 55-7 having the second smallest reduction gear ratio ispositioned nearer the left side. Each of the one-speed shift-gear pair55-1 having the largest reduction gear ratio and the two-speedshift-gear pair 55-2 having the second largest reduction gear ratio isdisposed on each of both ends of the arrangement of the eight shift-gearpairs 55-1 to 55-8. FIG. 4 illustrates an exemplary configuration wherethe one-speed shift-gear pair 55-1 having the largest reduction gearratio is disposed on the right end of the arrangement, and the two-speedshift-gear pair 55-2 having the second largest reduction gear ratio isdisposed on the left end of the arrangement.

The three-speed shift-gear pair 55-3 is disposed as being adjacent to aleft side, which is an opposite side of a side adjacent to theeight-speed shift-gear pair 55-8, with respect to the seven-speedshift-gear pair 55-7. The four-speed shift-gear pair 55-4 is disposed asbeing adjacent to a right side, which is an opposite side of a sideadjacent to the seven-speed shift-gear pair 55-7, with respect to theeight-speed shift-gear pair 55-8. Thus, a shift-gear pair (theeight-speed shift-gear pair 55-8) having the smallest reduction gearratio and a shift-gear pair (the seven-speed shift-gear pair 55-7)having the second smallest reduction gear ratio are adjacent one anotherat the center portion of the arrangement of the eight shift-gear pairs55-1 to 55-8. And, to opposite sides of the side where these shift-gearpairs are adjacent one another, shift-gear pairs where the differencesof the number of shift stages are four (the respective four-speedshift-gear pair 55-4 and three-speed shift-gear pair 55-3) are adjacent.

The five-speed shift-gear pair 55-5 is disposed adjacent to theone-speed shift-gear pair 55-1. The six-speed shift-gear pair 55-6 isdisposed adjacent to the two-speed shift-gear pair 55-2. Thus, theone-speed shift-gear pair 55-1 having the largest reduction gear ratioand the two-speed shift-gear pair 55-2 having the second largestreduction gear ratio are disposed on the respective both ends of thearrangement of the eight shift-gear pairs 55-1 to 55-8 and both areadjacent to the shift-gear pairs where the differences of the number ofshift stages are four.

Such configuration can suppress deformation of the counter shaft 51 andthe drive shaft 52. That is, a configuration where the arrangement ofthe eight drive gears 53-1 to 53-8 is disposed between the two bearings511R and 511L is likely to be deformed most at a center position betweenthe two bearings 511R and 511L when a force perpendicular to the axialdirection is applied to the counter shaft 51 (A deformation amount islikely to increase). Then, when ensuring the further multistage in thetransmission mechanism 5, a distance between the two bearings 5118 and511L increases corresponding to increase of the total number of shiftstages, thus the counter shaft 51 becomes easily to be deformed. Thesame applies to the drive shaft 52. On the other hand, if outputs of therotative power transmitted from the counter shaft 51 to the drive shaft52 are identical, the larger the reduction gear ratios of the shift-gearpairs 55-1 to 55-8 are, the larger torque is, thus burdens (reactiveforces) applied to the drive gears 53-1 to 53-8 and the driven gears54-1 to 54-8 become large. In view of this, force (force perpendicularto the axial direction) being applied to the counter shaft 51 and thedrive shaft 52 for pushing away one another becomes smallest at aposition where a shift-gear pair having the smallest reduction gearratio is disposed, and becomes largest at a position where a shift-gearpair having the largest reduction gear ratio is disposed.

Therefore, the eight-speed shift-gear pair 55-8 having the smallestreduction gear ratio and the seven-speed shift-gear pair 55-7 having thesecond smallest reduction gear ratio are disposed as being adjacent oneanother at the center portion of the arrangement of the eight shift-gearpairs 55-1 to 55-8. That is, the eight-speed drive gear 53-8 and theseven-speed drive gear 53-7 are disposed at positions (positions closestto the center position between the two bearings 511R and 511L) farthestfrom the two bearings 511R and 511L, which support the counter shaft 51.Similarly, the eight-speed driven gear 54-8 and the seven-speed drivengear 54-7 are disposed at positions farthest from the two bearings 521Rand 521L, which support the drive shaft 52. Such configuration canreduce force (force distancing one another) applied to a position thatis most likely to be deformed for each of the counter shaft 51 and thedrive shaft 52. This can suppress the deformation of the counter shaft51 and the drive shaft 52 even when the total number of shift stages ofthe transmission mechanism 5 is increased from conventional and generalsix-speed to eight-speed as the embodiment. Accordingly, suppression ofstrength decreases of the counter shaft 51 and the drive shaft 52, andsuppression of decreased sensation of shifting operation by the occupantcan be ensured.

Each of the one-speed shift-gear pair 55-1 having the largest reductiongear ratio and the two-speed shift-gear pair 55-2 having the secondlargest reduction gear ratio is disposed on each of both ends of thearrangement of the eight shift-gear pairs 55-1 to 55-8. Both ends of thearrangement of the eight shift-gear pairs 55-1 to 55-8 are positionsclosest to the bearings 511R and 511L, or 521R and 521L on any of thecounter shaft 51 and the drive shaft 52. In view of this, theconfiguration where the one-speed shift-gear pair 55-1 having thelargest reduction gear ratio and the two-speed shift-gear pair 55-2having the second largest reduction gear ratio are disposed on thepositions closest to the respective bearings 511R and 511L, and 521R and521L can suppress the deformation in directions perpendicular to theaxial directions of the counter shaft 51 and the drive shaft 52.

Thus, the embodiment can suppress the deformation of the counter shaft51 and the drive shaft 52. Accordingly, the embodiment ensures thefurther multistage of the transmission mechanism 5 while suppressing thedeformation of the counter shaft 51 and the drive shaft 52. Especially,the embodiment can increase the total number of shift stages of thetransmission mechanism 5 from conventional and general six-speed toeight-speed while suppressing the deformation of the counter shaft 51and the drive shaft 52. The suppression of strength decreases of thecounter shaft 51 and the drive shaft 52, and the suppression ofdecreased sensation of shifting operation by the occupant can beensured.

The following describes the arrangements of the eight drive gears 53-1to 53-8 and the eight driven gears 54-1 to 54-8. As described above, theeight drive gears 53-1 to 53-8 include the two fixed gears, the twoslide gears, and the four free gears. The eight driven gears 54-1 to54-8 also include the two fixed gears, the two slide gears, and the fourfree gears.

Among the eight drive gears 53-1 to 53-8, the free gears are applied tothe seven-speed drive gear 53-7 and the eight-speed drive gear 53-8,which are adjacent one another at the center portion of the arrangement.The slide gears are applied to the three-speed drive gear 53-3 and thefour-speed drive gear 53-4, which are adjacent to the opposite sides ofthe sides where the seven-speed drive gear 53-7 and the eight-speeddrive gear 53-8 are adjacent one another. The fixed gears are applied tothe one-speed drive gear 53-1 and the two-speed drive gear 53-2, whichare positioned at both ends of the arrangement. The free gears areapplied to the five-speed drive gear 53-5 and the six-speed drive gear53-6, which are adjacent to the one-speed drive gear 53-1 and thetwo-speed drive gear 53-2 respectively.

Among the eight driven gears 54-1 to 54-8, the fixed gears are appliedto the seven-speed driven gear 54-7 and the eight-speed driven gear54-8, which are adjacent one another at the center portion of thearrangement. The free gears are applied to the three-speed driven gear54-3 and the four-speed driven gear 54-4, which are adjacent to theopposite sides of the side where the seven-speed driven gear 54-7 andthe eight-speed driven gear 54-8 are adjacent one another. The freegears are applied to the one-speed driven gear 54-1 and the two-speeddriven gear 54-2, which are positioned at both ends of the arrangement.The slide gears are applied to the five-speed driven gear 54-5 and thesix-speed driven gear 54-6, which are adjacent to the one-speed drivengear 54-1 and the two-speed driven gear 54-2 respectively.

Thus, on the counter shaft 51, types of the four drive gears disposed ata right side of the arrangement center C and types of the four drivegears disposed at a left side of the arrangement center C aresymmetrical arrangements with respect to the arrangement center C.Similarly, on the drive shaft 52, types of the four driven gearsdisposed at the right side of the arrangement center C and types of thefour driven gears disposed at the left side of the arrangement center Care symmetrical arrangements with respect to the arrangement center C.

The eight drive gears 53-1 to 53-8 are separate gears one another.Similarly, the eight driven gears 54-1 to 54-8 are also separate gearsone another. That is, an integrated formation (a composite gear) of twoor more gears adjacent in the axial direction is not applied to any ofthe eight drive gears 53-1 to 53-8 and the eight driven gears 54-1 to54-8. Such configuration can facilitate production of the eight drivegears 53-1 to 53-8 and the eight driven gears 54-1 to 54-8. That is,when forming gear teeth, the integrated formation of two or more gearsadjacent in the axial direction makes it difficult to sufficientlyensure an intersecting angle with a shaving cutter in order to avoidinterference with the adjacent gears when forming gear teeth. This makesit difficult to ensure accuracy of tooth forms of the gears. Incontrast, according to the embodiment, the eight drive gears 53-1 to53-8 and the eight driven gears 54-1 to 54-8 are all separate, thusfacilitating ensuring the accuracy of the tooth forms in production tofacilitate the production.

FIG. 5 is a table that summarizes the arrangement of the eightshift-gear pairs 55-1 to 55-8 and the types of the drive gears 53-1 to53-8 and the driven gears 54-1 to 54-8, which form the eight respectiveshift-gear pairs 55-1 to 55-8. “DISPOSITION” in the table indicates anorder from the end of the arrangement of the shift-gear pairs 55-1 to55-8. However, this order may be from any of the right end and the leftend. Terms in parentheses of an item of the shift stage in the tableindicate shift stages corresponding to the example illustrated in FIG.4.

As illustrated in FIG. 5, the eight-speed shift-gear pair 55-8 havingthe smallest reduction gear ratio and the seven-speed shift-gear pair55-7 having the second smallest reduction gear ratio are disposed asbeing adjacent one another at the center portion of the arrangement ofthe eight shift-gear pairs 55-1 to 55-8. The drive gears 53-8 and 53-7and the driven gears 54-8 and 54-8 form these eight-speed shift-gearpair 55-8 and seven-speed shift-gear pair 55-7 respectively. The freegears are applied to the drive gears 53-8 and 53-7. The fixed gears areapplied to the driven gears 54-8 and 54-7. The four-speed shift-gearpair 55-4 and the three-speed shift-gear pair 55-3, whose numbers ofshift stages are different from those of the eight-speed shift-gear pair55-8 and the seven-speed shift-gear pair 55-7 by four respectively, aredisposed at the opposite sides (At the opposite sides of the arrangementcenter C. Third and sixth.) of the side where the eight-speed shift-gearpair 55-8 and the seven-speed shift-gear pair 55-7 are adjacent oneanother. The drive gears 53-4 and 53-3 and the driven gears 54-4 and54-3 form the four-speed shift-gear pair 55-4 and the three-speedshift-gear pair 55-3 respectively. The slide gears are applied to thedrive gears 53-4 and 53-3. The free gears are applied to the drivengears 54-4 and 54-3.

Each of the one-speed shift-gear pair 55-1 having the largest reductiongear ratio and the two-speed shift-gear pair 55-2 having the secondlargest reduction gear ratio is disposed on each of both ends (first andeighth) of the arrangement of the eight shift-gear pairs 55-1 to 55-8.The drive gears 53-1 and 53-2 and the driven gears 54-1 and 54-2 formthe one-speed shift-gear pair 55-1 and the two-speed shift-gear pair55-2 respectively. The fixed gears are applied to the drive gears 53-1and 53-2. The free gears are applied to the driven gears 54-1 and 54-2.Each of the five-speed shift-gear pair 55-5 and the six-speed shift-gearpair 55-6, whose numbers of shift stages are different from those of theone-speed shift-gear pair 55-1 and the two-speed shift-gear pair 55-2 byfour, is disposed at a position (second and seventh) adjacent to each ofthese one-speed shift-gear pair 55-1 and two-speed shift-gear pair 55-2.The drive gears 53-5 and 53-6 and the driven gears 54-5 and 54-6 formthe five-speed shift-gear pair 55-5 and the six-speed shift-gear pair55-6 respectively. The free gears are applied to the drive gears 53-5and 53-6. The slide gears are applied to the driven gears 54-5 and 54-6.

Thus, the types of the eight drive gears 53-1 to 53-8 disposed on thecounter shaft 51 are symmetric about the arrangement center C. Forexample, in the example illustrated in FIG. 5, from the arrangementcenter C to the right and left sides, the free gears (the fourtheight-speed drive gear 53-8 and the fifth seven-speed drive gear 53-7),the slide gears (the third four-speed drive gear 53-4 and the sixththree-speed drive gear 53-3), the free gears (the second five-speeddrive gear 53-5 and the seventh six-speed drive gear 53-6), and thefixed gears (the first one-speed drive gear 53-1 and the eighthtwo-speed drive gear 53-2) are arranged in this order.

The types of the eight driven gears 54-1 to 54-8 disposed on the driveshaft 52, similar to those of the eight drive gears 53-1 to 53-8disposed on the counter shaft 51, are also disposed as being arrangedsymmetrically with respect to the arrangement center C. For example, inthe example illustrated in FIG. 5, from the center position of thearrangement of the eight driven gears 54-1 to 54-8 to the respective ofright and left sides, the fixed gears (the fourth eight-speed drivengear 54-8 and the fifth seven-speed driven gear 54-7), the free gears(the third four-speed driven gear 54-4 and the sixth three-speed drivengear 54-3), the slide gears (the second five-speed driven gear 54-5 andthe seventh six-speed driven gear 54-6), and the free gears (the firstone-speed driven gear 54-1 and the eighth two-speed driven gear 54-2)are arranged in this order.

As illustrated in FIG. 5, the eight drive gears 53-1 to 53-8 and theeight driven gears 54-1 to 54-8 each include the two slide gears. Thefree gears are each adjacent to both sides in the axial direction ofthese slide gears. In the example (the parts in parentheses in FIG. 5)illustrated in FIG. 4, the three-speed drive gear 53-3 is the slidegear, and the six-speed drive gear 53-6 and the seven-speed drive gear53-7, which are adjacent to the three-speed drive gear 53-3, are thefree gears. The four-speed drive gear 53-4 is the slide gear, and thefive-speed drive gear 53-5 and the eight-speed drive gear 53-8, whichare adjacent to the four-speed drive gear 53-4, are the free gears. Thefive-speed driven gear 54-5 is the slide gear, and the one-speed drivengear 54-1 and the four-speed driven gear 54-4, which are adjacent to thefive-speed driven gear 54-5, are the free gears. The six-speed drivengear 54-6 is the slide gear, and the two-speed driven gear 54-2 and thethree-speed driven gear 54-3, which are adjacent to the six-speed drivengear 54-6, are the free gears.

Thus, the transmission mechanism 5 includes four sets of gearsconstituted of one slide gear and two free gears, which are adjacent tothe slide gear. For convenience of explanation, “a set of gearsconstituted of one slide gear and two free gears, which are adjacent tothe slide gear,” is referred to as “a set of slide/free gears.” In eachof the four sets of slide/free gears, addendum circle diameters (outsidediameters) of the two free gears are different one another, and anaddendum circle diameter (an outside diameter) of the one slide gear issmaller than any of the addendum circle diameters (the outsidediameters) of the two free gears. Thus, in each of the four sets, theone slide gear is disposed between the two free gears, whose addendumcircle diameters are larger than that of the one slide gear. In each ofthe sets of slide/free gears, among differences of the numbers of shiftstages between the one slide gear and the respective two free gears, thelarger one (that is, the difference of the number of shift stages fromthe free gear having the large diameter) is four, which is one half ofthe total number of shift stages of the transmission mechanism 5.

In the example illustrated in FIG. 4, the four-speed drive gear 53-4,which is the slide gear, is adjacent to the five-speed drive gear 53-5and the eight-speed drive gear 53-8, which are the free gears. The largedifference of the number of shift stages is four, and the smalldifference of the number of shift stages is one. The three-speed drivegear 53-3, which is the slide gear, is adjacent to the six-speed drivegear 53-6 and the seven-speed drive gear 53-7, which are the free gears.The large difference of the number of shift stages is four, and thesmall difference of the number of shift stages is three. The five-speeddriven gear 54-5, which is the slide gear, is adjacent to the one-speeddriven gear 54-1 and the four-speed driven gear 54-4, which are the freegears. The large difference of the number of shift stages is four, andthe small difference of the number of shift stages is one. The six-speeddriven gear 54-6, which is the slide gear, is adjacent to the two-speeddriven gear 54-2 and the three-speed driven gear 54-3, which are thefree gears. The large difference of the number of shift stages is four,and the small difference of the number of shift stages is three.

The counter shaft 51 and the drive shaft 52 each include the two sets ofslide/free gears. Addendum circle diameters of the two free gears ineach set of the slide/free gears are different one another. Then,amplitude of the addendum circle diameters of the free gears in the twosets of slide/free gears disposed on each of the counter shaft 51 andthe drive shaft 52 is symmetric about the arrangement center C.

Specifically, for both of the two sets of slide/free gears disposed onthe counter shaft 51, the free gear having the large addendum circlediameter is disposed at a side close to the arrangement center C, andthe free gear having the small addendum circle diameter is disposed at aside far from the arrangement center C. In the example illustrated inFIG. 4, the eight-speed drive gear 53-8, which is the free gear havingthe large diameter, is adjacent to the four-speed drive gear 53-4, whichis the slide gear, at a side close to the arrangement center C, and thefive-speed drive gear 53-5, which is the free gear having the smalldiameter, is adjacent to the four-speed drive gear 53-4 at a side farfrom the arrangement center C. The seven-speed drive gear 53-7, which isthe free gear having the large diameter, is adjacent to the three-speeddrive gear 53-3, which is the slide gear, at a side close to thearrangement center C, and the six-speed drive gear 53-6, which is thefree gear having the small diameter, is adjacent to the three-speeddrive gear 53-3 at a side far from the arrangement center C.

Similarly, for both of the two sets of slide/free gears disposed on thedrive shaft 52, the free gear having the large addendum circle diameteris disposed at a side far from the arrangement center C, and the freegear having the small addendum circle diameter is disposed at a sideclose to the arrangement center C. In the example illustrated in FIG. 4,the one-speed driven gear 54-1, which is the free gear having the largediameter, is adjacent to the five-speed driven gear 54-5, which is theslide gear, at a side far from the arrangement center C, and thefour-speed driven gear 54-4, which is the free gear having the smalldiameter, is adjacent to the five-speed driven gear 54-5 at a side closeto the arrangement center C. The two-speed driven gear 54-2, which isthe free gear having the large diameter, is adjacent to the six-speeddriven gear 54-6, which is the slide gear, at a side far from thearrangement center C, and the three-speed driven gear 54-3, which is thefree gear having the small diameter, is adjacent to the six-speed drivengear 54-6 at a side close to the arrangement center C.

The following describes an exemplary configuration of the set ofslide/free gears with reference to FIG. 6A and FIG. 6B. FIG. 6A and FIG.6B are cross-sectional views schematically illustrating an exemplaryconfiguration of the set of the four-speed drive gear 53-4, which is theslide gear, and two free gears: the five-speed drive gear 53-5 and theeight-speed drive gear 53-8, which are adjacent to the four-speed drivegear 53-4, as an example of the set of slide/free gears.

As illustrated in FIG. 6A and FIG. 6B, the eight-speed drive gear 53-8(the free gear) is adjacent to the tour-speed drive gear 53-4 (the slidegear) at the side close to the arrangement center C. The five-speeddrive gear 53-5 (the free gear) is adjacent to the four-speed drive gear53-4 at the side far from the arrangement center C.

The four-speed drive gear 53-4, which is the slide gear, has an addendumcircle diameter smaller than addendum circle diameters of the five-speeddrive gear 53-5 and the eight-speed drive gear 53-8, which are the twofree gears adjacent to the four-speed drive gear 53-4. The addendumcircle diameters of the five-speed drive gear 53-5 and the eight-speeddrive gear 53-8 are different one another. In the combinationillustrated in FIG. 6A and FIG. 6B, the eight-speed drive gear 53-8 hasa diameter larger than that of the five-speed drive gear 53-5. Among endsurfaces of the four-speed drive gear 53-4, on an end surface at theside close to the eight-speed drive gear 53-8, which is the free gearhaving the large diameter, a predetermined number of convex portions(male dogs) for coupling to the eight-speed drive gear 53-8 are disposedas being arranged in a circumferential direction. For convenience ofexplanation, these convex portions are referred to asmain-body-portion-side convex portions 533-4. Thesemain-body-portion-side convex portions 533-4 project to the side of theeight-speed drive gear 53-8 from an end surface of a part (what iscalled a rim) where the gear teeth are disposed.

The four-speed drive gear 53-4, which is the slide gear, includes anengagement groove 535-4 where the shift fork 57 is engaged.Specifically, among the end surfaces of the four-speed drive gear 53-4,on an end surface at the side close to the five-speed drive gear 53-5,which is the free gear having the small diameter, a cylindrically-shapedpart (hereinafter referred to as “a tubular portion 534-4”), whichprojects to the side of the five-speed drive gear 53-5 from a part (arim) where the gear teeth are disposed, is disposed. The toricengagement groove 535-4 is disposed on an outer periphery of thistubular portion 534-4. Thus, the tubular portion 534-4 projecting in theaxial direction is disposed on the four-speed drive gear 53-4, which isthe slide gear, at an opposite side of the side close to the eight-speeddrive gear 53-8, which is the free gear having the large diameter. Theengagement groove 535-4 where the shift fork 57 is engaged is disposedon this tubular portion 534-4. Then, a predetermined number of convexportions for coupling to the five-speed drive gear 53-5 are disposed asbeing arranged in a circumferential direction on the end surface (theend surface at the side close to the five-speed drive gear 53-5) of thetubular portion 534-4. For convenience of explanation, these convexportions disposed on the tubular portion 534-4 are referred to astubular-portion-side convex portions 536-4.

A first hollow portion 531-8 and a predetermined number of second hollowportions 532-8 (female dogs) are disposed on an end surface (an endsurface at a side close to the four-speed drive gear 53-4) at a side farfrom the arrangement center C of the eight-speed drive gear 53-8, whichis the tree gear having the large diameter. The first hollow portion531-8 is a part where an end portion in the axial direction of the partwhere the gear teeth of the four-speed drive gear 53-4 are disposed canbe inserted. For example, a circular-shaped hollow portion, which has adiameter larger than the addendum circle diameter (the outside diameter)of the four-speed drive gear 53-4 and is coaxial to the counter shaft51, viewed from the axial direction of the counter shaft 51, isapplicable to the first hollow portion 531-8. The predetermined numberof second hollow portions 532-8 (the female dogs), which are parts wherethe respective predetermined number of main-body-portion-side convexportions 533-4 (the male dogs) of the four-speed drive gear 53-4 can fitinto, are disposed as being arranged in a circumferential direction.

FIG. 6B is a drawing illustrating a state where the four-speed drivegear 53-4 has moved to the eight-speed drive gear 53-8 side (the sideclose to the arrangement center C). As illustrated in FIG. 6B, if thefour-speed drive gear 53-4 moves to the eight-speed drive gear 53-8side, the end portion in the axial direction of the four-speed drivegear 53-4 gets into the first hollow portion 531-8 of the eight-speeddrive gear 53-8. Furthermore, the main-body-portion-side convex portions533-4 of the four-speed drive gear 53-4 get into the second hollowportions 532-8 of the eight-speed drive gear 53-8. In this state, fromthe side view (viewed from a direction perpendicular to the axialdirection of the counter shaft 51), a part of the four-speed drive gear53-4 and a part of the eight-speed drive gear 53-8 are superimposed oneanother. Especially, a part of the part where the gear teeth of thefour-speed drive gear 53-4 are disposed is superimposed on theeight-speed drive gear 53-8. A part illustrated by reference numeral Ain FIG. 6B is the part where the four-speed drive gear 53-4 and theeight-speed drive gear 53-8 are superimposed one another. Then, themain-body-portion-side convex portions 533-4 (the male dogs) of thefour-speed drive gear 53-4 get into the second hollow portions 532-8(the female dogs) of the eight-speed drive gear 53-8, thus making astate where the eight-speed drive gear 53-8 and the four-speed drivegear 53-4 are coupled together as integrally rotating.

A first hollow portion 531-5 and a predetermined number of second hollowportions 532-5 are disposed on an end surface (an end surface at a sideclose to the four-speed drive gear 53-4, which is the slide gear) at aside close to the arrangement center C of the five-speed drive gear53-5, which is the free gear having the small diameter. The first hollowportion 531-5 is a part that can house an end portion in the axialdirection of the tubular portion 534-4 of the four-speed drive gear53-4. For example, a circular-shaped hollow portion, which has adiameter larger than an outside diameter of the tubular portion 534-4 ofthe four-speed drive gear 53-4 and is coaxial to the counter shaft 51,viewed from the axial direction of the counter shaft 51, is applicableto the first hollow portion 531-5. The predetermined number of secondhollow portions 532-5 (the female dogs), which are parts where therespective predetermined number of tubular-portion-side convex portions536-4 (the male dogs) of the four-speed drive gear 53-4 can fit into,are disposed as being arranged in a circumferential direction.

The four-speed drive gear 53-4 has moved to the five-speed drive gear53-5 side (the side far from the arrangement center C), thus making astate where the end portion in the axial direction of the tubularportion 534-4 of the four-speed drive gear 53-4 has got into the firsthollow portion 531-5 of the five-speed drive gear 53-5. This makes astate where the tubular-portion-side convex portions 536-4 of thefour-speed drive gear 53-4 have got into the second hollow portions532-5 of the five-speed drive gear 53-5. In this state, from the sideview, a part of the tubular portion 534-4 of the four-speed drive gear53-4 and a part of the five-speed drive gear 53-5 are superimposed oneanother. The tubular-portion-side convex portions 536-4 (the male dogs)of the four-speed drive gear 53-4 get into the second hollow portions532-5 (the female dogs) of the five-speed drive gear 53-5, thus making astate where the five-speed drive gear 53-5 and the four-speed drive gear53-4 are coupled together as integrally rotating.

Thus, the configuration where the first hollow portion 531-8 is disposedon the eight-speed drive gear 53-8 allows the eight-speed drive gear53-8 and the four-speed drive gear 53-4 to be closely disposed in theaxial direction. This ensures suppression of increasing in size in theaxial direction of the transmission mechanism 5, or ensures reduction insize in the axial direction of the transmission mechanism 5. Similarly,the configuration where the first hollow portion 531-5 is disposed onthe five-speed drive gear 53-5 allows the four-speed drive gear 53-4 andthe five-speed drive gear 53-5 to be closely disposed.

The transmission mechanism 5 includes a set ofthree-speed/six-speed/seven-speed drive gears, a set ofone-speed/four-speed/five-speed driven gears, and a set oftwo-speed/three-speed/six-speed driven gears, in addition to theabove-described set of four-speed/five-speed/eight-speed drive gears, asthe set of slide/free gears. The set ofthree-speed/six-speed/seven-speed drive gears, the set ofone-speed/four-speed/five-speed driven gears, and the set oftwo-speed/three-speed/six-speed driven gears all have configurationssimilar to that of the above-described set offour-speed/five-speed/eight-speed drive gears.

Thus, in all the set of slide/free gears, the slide gear and therespective of two free gears, which are adjacent to the slide gear, arerotatably coupled together, such that the convex portions (themain-body-portion-side convex portions and the tubular-portion-sideconvex portions) of the slide gear get into the hollow portions (thesecond hollow portions) of the free gears. Then, themain-body-portion-side convex portions, which project in the axialdirection from the end surface of the part (the rim) where the gearteeth are disposed is disposed on the slide gear. In contrast, thehollow portions (the first hollow portion and the second hollowportions), which are depressed from the end surface of the part (therim) where the gear teeth are disposed, is disposed on the free gear.However, convex portions, which project from the end surface of the partwhere the gear teeth are disposed to the slide gear side, is notdisposed on the free gear. Thus, the slide gear and the free gear do nothave a configuration where the convex portions (the male dogs) areengaged one another. The slide gear and the free gear have aconfiguration where the convex portions (the male dogs) disposed on onehand is engaged with the hollow portions (the female dogs) disposed onthe other hand. Then, a structure (the hollow portion) disposed on thefree gear for coupling to the slide gear so as to ensure thetransmission of the rotative power is disposed at a position that doesnot project to the slide gear side from the part where the gear teethare disposed on the free gear. Such configuration ensures reduction of adistance in the axial direction between the slide gear and the free gearto reduce a dimension in the lateral direction of the transmissionmechanism 5.

In the set of slide/free gears, the addendum circle diameter of the oneslide gear is smaller than the addendum circle diameters of the two freegears. Then, the first hollow portion where the end portion in the axialdirection of the one slide gear can fit into is disposed on the endsurface of the side being opposed to the one slide gear, of the freegear having the large diameter among the two free gears. In the statewhere the one slide gear and the free gear having the large diameter arecoupled together as integrally rotating, from the side view (viewed fromthe direction perpendicular to the axial direction), a part of the oneslide gear and a part of the free gear having the large diameter aresuperimposed one another. Especially, the end portion in the axialdirection of the part (what is called a rim) where the gear teeth aredisposed, on the one slide gear, is superimposed on the free gear havingthe large diameter (the part illustrated by reference numeral A in FIG.6B). Such configuration ensures reduction of a distance between the oneslide gear and the free gear having the large diameter among the twofree gears in each of the sets of slide/free gears.

In each of the four sets of slide/free gears, a difference in the numberof shift stages between the slide gear and the free gear having thelarge diameter is four. This difference in the number of shift stages isone half of eight, which is the total number of shift stages of thetransmission mechanism 5. In any set of slide/free gears, the differencein the number of shift stages between the slide gear and the free gearhaving the large diameter is larger than a difference in the number ofshift stages between the slide gear and the free gear having the smalldiameter. Such configuration ensures increasing a difference in theaddendum circle diameters between the slide gear and the free gearhaving the large diameter, thus ensuring the disposition of the firsthollow portion where the slide gear can fit into the free gear havingthe large diameter, in all the sets of slide/free gears. Accordingly,this ensures the reduction in size of the transmission mechanism 5.

For the set of four-speed/five-speed/eight-speed drive gears and the setof three-speed/six-speed/seven-speed drive gears, which are disposed onthe counter shaft 51, directions of the slide gear and the free gears,which are included in these sets, are symmetric about the arrangementcenter C. Specifically, the first hollow portion 531-8 and the secondhollow portions 532-8 of the eight-speed drive gear 53-8, and a firsthollow portion 531-7 and second hollow portions 532-7 of the seven-speeddrive gear 53-7 are all disposed on end surfaces at sides far from thearrangement center C. The main-body-portion-side convex portions 533-4of the four-speed drive gear 53-4 and main-body-portion-side convexportions 533-3 of the three-speed drive gear 53-3 are both disposed onend surfaces at sides close to the arrangement center C. The tubularportion 534-4 and the tubular-portion-side convex portions 536-4 of thefour-speed drive gear 53-4, and a tubular portion 534-3 andtubular-portion-side convex portions 536-3 of the three-speed drive gear53-3 are all disposed on end surfaces at sides far from the arrangementcenter C. The first hollow portion 531-5 and the second hollow portions532-5 of the five-speed drive gear 53-5, and a first hollow portion531-6 and second hollow portions 532-6 of the six-speed drive gear 53-6are all disposed on end surfaces at sides close to the arrangementcenter C.

The same applies to the set of one-speed/four-speed/five-speed drivengears and the set of two-speed/three-speed/six-speed driven gears, whichare disposed on the drive shaft 52. That is, a first hollow portion541-1 and second hollow portions 542-1 of the one-speed driven gear54-1, and a first hollow portion 541-2 and second hollow portions 542-2of the two-speed driven gear 54-2 are all disposed on end surfaces atsides close to the arrangement center C. Main-body-portion-side convexportions 543-5 of the five-speed driven gear 54-5 andmain-body-portion-side convex portions 543-6 of the six-speed drivengear 54-6 are both disposed on end surfaces at sides far from thearrangement center C. A tubular portion 544-5 and tubular-portion-sideconvex portions 546-5 of the five-speed driven gear 54-5, and a tubularportion 544-6 and tubular-portion-side convex portions 546-6 of thesix-speed driven gear 54-6 are all disposed on end surfaces at sidesclose to the arrangement center C. A first hollow portion 541-4 andsecond hollow portions 542-4 of the four-speed driven gear 54-4, and afirst hollow portion 541-3 and second hollow portions 542-3 of thethree-speed driven gear 54-3 are all disposed on end surfaces at sidesfar from the arrangement center C.

Furthermore, disposition of the free gear having the large diameter andthe free gear having the small diameter, which are included in the setsof slide/free gears are also symmetric about the arrangement center C.That is, for both of the set of four-speed/five-speed/eight-speed drivegears and the set of three-speed/six-speed/seven-speed drive gears,which are disposed on the counter shaft 51, the free gears (theeight-speed drive gear 53-8 and the seven-speed drive gear 53-7) havingthe large diameters are disposed at the sides close to the arrangementcenter C, and the free gears (the five-speed drive gear 53-5 and thesix-speed drive gear 53-6) having the small diameters are disposed atthe sides far from the arrangement center C. For both of the set ofone-speed/four-speed/five-speed driven gears and the set oftwo-speed/three-speed/six-speed driven gears, which are disposed on thedrive shaft 52, the free gears (the one-speed driven gear 54-1 and thetwo-speed driven gear 54-2) having the large diameters are disposed atthe side far from the arrangement center C, and the free gears (thefour-speed driven gear 54-4 and the three-speed driven gear 54-3) havingthe small diameters are disposed at the sides close to the arrangementcenter C.

The four free gears included in the eight drive gears 53-1 to 53-8 eachhave a configuration where the first hollow portion and the secondhollow portions are disposed on one end surface in the axial direction.The two slide gears included in the eight drive gears 53-1 to 53-8 eachhave a configuration where the main-body-portion-side convex portionsare disposed on one end surface, and the tubular portion and thetubular-portion-side convex portions are disposed on another endsurface, in the axial direction. The hollow portion, the convex portion,and similar portion are not disposed on any of the two fixed gearsincluded in the eight drive gears 53-1 to 53-8. The same applies to theeight driven gears 54-1 to 54-8. Thus, the embodiment has similar shapes(in other words, common shapes) by free gear, by slide gear, and byfixed gear. With such configuration, all the transmission gears (thedrive gears and the driven gears) are separately formed as describedabove, and each transmission gear includes one series of gear teeth,thus facilitating processing of the gear teeth. Aligning the shape ofeach free gear such that only the hollow portions are disposed on onlythe one surface, and the convex portions are not disposed ensurescommunalization of processing operations and production facilities toimprove productivity. Furthermore, each slide gear has the configurationwhere only the convex portions are disposed, and the hollow portions arenot disposed, thus similarly ensuring the communalization of processingoperations and production facilities to improve the productivity.

Here, an exemplary disposition of the shift forks 57 will be describedwith reference to FIG. 7. FIG. 7 is a cross-sectional view schematicallyillustrating the exemplary disposition of the shift forks 57. Asillustrated in FIG. 7, the counter shaft 51 and the drive shaft 52 eachinclude the two slide gears. Then, one shift fork guide 58 among the twoshift fork guides 58 slidably and reciprocatably supports two shiftforks 57, which each move each of the two slide gears (the three-speeddrive gear 53-3 and the four-speed drive gear 53-4) disposed on thecounter shaft 51. Similarly, another shift fork guide 58 among the twoshift fork guides 58 slidably and reciprocatably supports two shiftforks 57, which each move each of the two slide gears (the five-speeddriven gear 54-5 and the six-speed driven gear 54-6) disposed on thedrive shaft 52. Thus, the one shift fork guide 58 among the two shiftfork guides 58 collectively supports the shift forks 57, which move theslide gears disposed on the counter shaft 51, and the other shift forkguide 58 collectively supports the shift forks 57, which move the slidegears disposed on the drive shaft 52.

Operation of Transmission Mechanism

The following describes an operation that the transmission mechanism 5shifts gears. As illustrated in FIG. 7, the two shift fork guides 58each reciprocatably support the two shift forks 57. The four cam grooves561 are disposed on the shift cam 56. Then, each of the two shift forks57, which are supported by the one shift fork guide 58, is engaged witheach of the two slide gears, which are disposed on the counter shaft 51,and is engaged with each of the four cam grooves 561, which are disposedon the shift cam 56. Similarly, each of the two shift forks 57, whichare supported by the other shift fork guide 58, is engaged with each ofthe two slide gears, which are disposed on the drive shaft 52, and isengaged with each of the four cam grooves 561, which are disposed on theshift cam 56. Then, corresponding to the rotation of the shift cam 56,each of the four shift forks 57 moves in the axial direction to moveeach of the four slide gears in the axial direction in an aspectdescribed later. This switches (shifts gears) the shift stage (the shiftposition) of the transmission mechanism 5.

Neutral Position (Neutral)

A neutral position is in a state where all the four slide gears are notcoupled to any of the two free gears, which are adjacent to any of thefour slide gears, so as to transmit the rotative power. Specifically,the neutral position is in a state where the three-speed drive gear 53-3is not coupled to any of the six-speed drive gear 53-6 and theseven-speed drive gear 53-7, the four-speed drive gear 53-4 is notcoupled to any of the five-speed drive gear 53-5 and the eight-speeddrive gear 53-8, the six-speed driven gear 54-6 is not coupled to any ofthe two-speed driven gear 54-2 and the three-speed driven gear 54-3, andthe five-speed driven gear 54-5 is not coupled to any of the four-speeddriven gear 54-4 and the one-speed driven gear 54-1.

In this state, rotation of the four-speed drive gear 53-4, which is theslide gear, is not transmitted to any of the eight-speed drive gear 53-8and the five-speed drive gear 53-5, which are the free gears adjacent tothe four-speed drive gear 53-4. Similarly, rotation of the three-speeddrive gear 53-3, which is the slide gear, is not transmitted to any ofthe seven-speed drive gear 53-7 and the six-speed drive gear 53-6, whichare the free gears adjacent to the three-speed drive gear 53-3. In viewof this, the rotative power of the counter shaft 51 is not transmittedto the drive shaft 52 via thefive-speed/six-speed/seven-speed/eight-speed shift-gear pairs 55-5 to55-8.

The five-speed driven gear 54-5, which is the slide gear, is not coupledto any of the one-speed driven gear 54-1 and the four-speed driven gear54-4, which are adjacent to the five-speed driven gear 54-5, thus theone-speed driven gear 54-1 and the four-speed driven gear 54-4 are instates that are rotatable relative to the drive shaft 52. Similarly, thesix-speed driven gear 54-6, which is the slide gear, is not coupled toany of the two-speed driven gear 54-2 and the three-speed driven gear54-3, which are adjacent to the six-speed driven gear 54-6, thus thetwo-speed driven gear 54-2 and the three-speed driven gear 54-3 are bothin states that are rotatable relative to the drive shaft 52. In view ofthis, the rotative power is not transmitted to the drive shaft 52 viathe one-speed/two-speed/three-speed/four-speed shift-gear pairs 55-1 to55-4. Accordingly, when the shift position is at the neutral position,the rotative power of the counter shaft 51 is not transmitted to thedrive shaft 52.

One-Speed

The five-speed driven gear 54-5 is moved from the neutral position to aside of the one-speed driven gear 54-1 to switch to a state where thefive-speed driven gear 54-5 is coupled to the one-speed driven gear54-1. This state is one-speed. In this state, the one-speed driven gear54-1 and the five-speed driven gear 54-5 integrally rotate. In view ofthis, the rotative power of the countershaft 51 is transmitted to thedrive shaft 52 via the one-speed shift-gear pair 55-1 (the one-speeddrive gear 53-1 and the one-speed driven gear 54-1) and the five-speeddriven gear 54-5.

Two-Speed

The five-speed driven gear 54-5 is moved from the state at one-speed toa position that is not coupled to any of the one-speed and four-speeddriven gears 54-1 and 54-4. In this state, the coupling between theone-speed driven gear 54-1 and the five-speed driven gear 54-5 has beenreleased, and the one-speed driven gear 54-1 is in a rotatable statewith respect to the drive shaft 52 to be in the state at the neutralposition. The six-speed driven gear 54-6 is moved from this neutralposition to a side of the two-speed driven gear 54-2 to switch to astate where the six-speed driven gear 54-6 is coupled to the two-speeddriven gear 54-2. This state is two-speed. In this state, the two-speeddriven gear 54-2 and the six-speed driven gear 54-6 integrally rotate.In view of this, the rotative power of the counter shaft 51 istransmitted to the drive shaft 52 via the two-speed shift-gear pair 55-2(the two-speed drive gear 53-2 and the two-speed driven gear 54-2) andthe six-speed driven gear 54-6.

Three-Speed

The six-speed driven gear 54-6 is moved from the state at two-speed to aside of the three-speed driven gear 54-3 to switch to a state where thesix-speed driven gear 54-6 is coupled to the three-speed driven gear54-3. This state is three-speed. In this state, the coupling between thetwo-speed driven gear 54-2 and the six-speed driven gear 54-6 has beenreleased, and the two-speed driven gear 54-2 is in a rotatable statewith respect to the drive shaft 52. The three-speed driven gear 54-3integrally rotates with the six-speed driven gear 54-6. In view of this,the rotative power of the counter shaft 51 is transmitted to the driveshaft 52 via the three-speed shift-gear pair 55-3 (the three-speed drivegear 53-3 and the three-speed driven gear 54-3) and the six-speed drivengear 54-6.

Four-Speed

The six-speed driven gear 54-6 is moved from the state at three-speed toa position that is not coupled to any of the three-speed driven gear54-3 and the two-speed driven gear 54-2. The five-speed driven gear 54-5is moved to a side of the four-speed driven gear 54-4 to switch to astate where the five-speed driven gear 54-5 is coupled to the four-speeddriven gear 54-4. This state is four-speed. In this state, the two-speeddriven gear 54-2 and the three-speed driven gear 54-3 both are inrotatable states with respect to the drive shaft 52. In view of this,the rotative power of the counter shaft 51 is not transmitted to thedrive shaft 52 via the two-speed shift-gear pair 55-2 and thethree-speed shift-gear pair 55-3. On the other hand, the four-speeddriven gear 54-4 is in a state that integrally rotates with thefive-speed driven gear 54-5. In view of this, the rotative power of thecounter shaft 51 is transmitted to the drive shaft 52 via the four-speedshift-gear pair 55-4 (the four-speed drive gear 53-4 and the four-speeddriven gear 54-4) and the five-speed driven gear 54-5.

Five-Speed

The five-speed driven gear 54-5 is moved from the state at four-speed tothe position that is not coupled to any of the one-speed and four-speeddriven gears 54-1 and 54-4. The four-speed drive gear 53-4 is moved tothe five-speed drive gear 53-5 side to switch to a state where thefour-speed drive gear 53-4 is coupled to the five-speed drive gear 53-5.This state is five-speed. In this state, the four-speed driven gear 54-4is in a rotatable state relative to the drive shaft 52, and thefive-speed drive gear 53-5 is in a state that integrally rotates withthe four-speed drive gear 53-4. In view of this, the rotative power ofthe counter shaft 51 is transmitted to the drive shaft 52 via thefour-speed drive gear 53-4 and the five-speed shift-gear pair 55-5 (thefive-speed drive gear 53-5 and the five-speed driven gear 54-5).

Six-Speed

The four-speed drive gear 53-4 is moved from the state at five-speed toa position that is not coupled to any of the five-speed drive gear 53-5and the eight-speed drive gear 53-8. The three-speed drive gear 53-3 ismoved to a side of the six-speed drive gear 53-6 to be coupled to switchto a state where the three-speed drive gear 53-3 is coupled to thesix-speed drive gear 53-6. This state is six-speed. In this state, thefive-speed drive gear 53-5 is in a rotatable state relative to thecounter shaft 51, and the six-speed drive gear 53-6 is in a state thatintegrally rotates with the three-speed drive gear 53-3. In view ofthis, the rotative power of the counter shaft 51 is transmitted to thedrive shaft 52 via the three-speed drive gear 53-3 and the six-speedshift-gear pair 55-6 (the six-speed drive gear 53-6 and the six-speeddriven gear 54-6).

Seven-Speed

The three-speed drive gear 53-3 is moved from the state at six-speed toa side of the seven-speed drive gear 53-7 to switch to a state where thethree-speed drive gear 53-3 is coupled to the seven-speed drive gear53-7. This state is seven-speed. In this state, the six-speed drive gear53-6 is in a rotatable state relative to the counter shaft 51, and theseven-speed drive gear 53-7 is in a state that integrally rotates withthe three-speed drive gear 53-3. In view of this, the rotative power ofthe counter shaft 51 is transmitted to the drive shaft 52 via thethree-speed drive gear 53-3 and the seven-speed shift-gear pair 55-7(the seven-speed drive gear 53-7 and the seven-speed driven gear 54-7).

Eight-Speed

The three-speed drive gear 53-3 is moved from the state at seven-speedto a position that is not coupled to any of the six-speed drive gear53-6 and the seven-speed drive gear 53-7. The four-speed drive gear 53-4is moved to the eight-speed drive gear 53-8 side to switch to a statewhere the four-speed drive gear 53-4 is coupled to the eight-speed drivegear 53-8. This state is eight-speed. In this state, the six-speed drivegear 53-6 and the seven-speed drive gear 53-7 both are in rotatablestates relative to the counter shaft 51. The eight-speed drive gear 53-8is in a state that integrally rotates with the four-speed drive gear53-4. In view of this, the rotative power of the counter shaft 51 istransmitted to the drive shaft 52 via the four-speed drive gear 53-4 andthe eight-speed shift-gear pair 55-8 (the eight-speed drive gear 53-8and the eight-speed driven gear 54-8).

As described above, rotating the shift cam 56 moves each of the fourslide gears as described above to ensure the switch (the shift of thegear) of the shift stage (the shift position).

As described above, the embodiment of the present invention has beendescribed in detail with reference to the drawings. However, theabove-described embodiment merely indicates a concrete example forexploitation of the present invention. The technical scope of thepresent invention is not limited to the above-described embodiment.Various modifications of the present invention can be made withoutdeparting from its spirit, such modifications being included within thetechnical scope of this invention.

For example, the above-described embodiment has indicated an examplewhere the transmission mechanism of the present invention is applied tothe engine unit of the motorcycle. However, the present invention is notlimited to such configuration. The present invention is also applicableto engine units of various saddle-ride type vehicles such as athree-wheeled vehicle and a four-wheeled vehicle, and various vehiclesother than the saddle-ride type vehicle, in addition to the motorcycle.The above-described embodiment has indicated the engine unit where thetransmission mechanism of the present invention is integrally attachedwith the driving force source (the engine portion). However, the presentinvention is not limited to such configuration. The transmissionmechanism of the present invention is also applicable to a transmissionindependent from the driving force source. The present invention isapplicable to the transmission mechanism insofar as it is aconstant-mesh type transmission mechanism.

The present invention is a technique effective for a constant-mesh typetransmission mechanism. Then, the present invention ensures the furthermultistage while suppressing the deformation of the counter shaft andthe drive shaft.

The present invention ensures the further multistage while suppressingthe deformation of the counter shaft and the drive shaft.

What is claimed is:
 1. A transmission mechanism comprising: an inputshaft to which a rotative power is transmitted; a plurality of drivegears arranged in an axial direction on the input shaft, wherein: theplurality of drive gears comprise slide gears reciprocatable in theaxial direction of the input shaft, and free gears disposed adjacent tothe slide gears in the axial direction of the input shaft and rotatablerelative to the input shaft, and convex portions projecting from endsurfaces in the axial direction of teeth are disposed on ones of theslide gears and the free gears included in the plurality of drive gearsand adjacent to one another, hollow portions depressing from endsurfaces in the axial direction of teeth are disposed on others, and theconvex portions are fitted into the hollow portions to couple the slidegears and the free gears included in the plurality of drive gears andadjacent one another so as to ensure the transmission of the rotativepower; an output shaft that outputs a rotative power outside; and aplurality of driven gears arranged in an axial direction on the outputshaft, wherein: the plurality of driven gears comprise slide gearsreciprocatable in the axial direction of the output shaft, and freegears disposed adjacent to the slide gears in the axial direction of theoutput shaft and rotatable relative to the output shaft, convex portionsprojection from end surfaces in the axial direction of teeth aredisposed on ones of the slide gears and the free gears included in theplurality of driven gears and adjacent one another, hollow portionsdepressing from end surfaces in the axial direction of teeth aredisposed on others, and the convex portions are fitted into the hollowportions to couple the slide gears and the free gears included in theplurality of driven gears and adjacent one another so as to ensure thetransmission of the rotative power, each of the plurality of drive gearsis engaged with each of the plurality of driven gears so as to ensuretransmission of the rotative power to form a plurality of shift-gearpairs, the plurality of shift-gear pairs having mutually differentreduction gear ratios, a first drive gear included in a first shift-gearpair and a second drive gear included in a second shift-gear pair areadjacent one another at a center portion of an arrangement of theplurality of drive gears, the first shift-gear pair having a smallestreduction gear ratio among the plurality of shift-gear pairs, and thesecond shift-gear having a small reduction gear ratio next to the firstshift-gear pair, and a first driven gear included in the firstshift-gear pair and a second driven gear included in the secondshift-gear pair are adjacent one another at a center portion of anarrangement of the plurality of driven gears.
 2. The transmissionmechanism according to claim 1, wherein: each of the plurality of slidegears included in the plurality of drive gears and the plurality ofdriven gears is adjacent to two free gears having mutually differentoutside diameters among the plurality of free gears, a hollow portion isdisposed on a free gear having a large diameter of the two free gears,the hollow portion ensuring insert of an end portion in an axialdirection of the slide gear adjacent to the free gear having the largediameter and when the free gear having the large diameter is coupled tothe slide gear adjacent to the free gear having the large diameter so asto ensure the transmission of the rotative power, a part of the freegear having the large diameter is superimposed on a part of the slidegear adjacent to the free gear having the large diameter, viewed from adirection perpendicular to a rotational center line.
 3. The transmissionmechanism according to claim 2, wherein differences of counts of shiftstages of the free gears having the large diameters and counts of shiftstages of the slide gears adjacent to the free gears having the largediameter are all identical.
 4. The transmission mechanism according toclaim 1, wherein: the plurality of drive gears further include aplurality of fixed gears that are nonmovable in the axial direction withrespect to the input shaft and integrally rotate with the input shaft,and the free gears, the slide gears, and the fixed gears included in theplurality of drive gears are symmetrically disposed about a center ofthe arrangement of the plurality of drive gears.
 5. The transmissionmechanism according to claim 1, wherein: the plurality of driven gearsfurther include a plurality of fixed gears that are nonmovable in theaxial direction with respect to the output shaft and integrally rotatewith the output shaft, and the free gears, the slide gears, and thefixed gears included in the plurality of driven gears are symmetricallydisposed about a center of the arrangement of the plurality of drivengears.